In many process engineering facilities, liquids are sprayed into a gaseous fluid, e.g. into a flue gas to be cleaned or cooled, hence for flue gas cleaning or for evaporative cooling. It is frequently of crucial importance here that the liquid is atomized into the finest possible droplets. The finer the droplets, the larger the specific droplet surface. Considerable process engineering advantages can be obtained as a result. For example, the size of a reaction vessel and its manufacturing costs depend crucially on the mean droplet size. But in many cases it is in no way sufficient for the mean droplet size to fall below a certain limit value. Even a few considerably larger droplets can cause considerable disruptions in operation. This is particularly the case when the droplets do not evaporate quickly enough due to their size, so that droplets or even doughy particles are deposited in the following components, e.g. onto fabric filter hoses or onto fan blades, leading to operating disruptions due to incrustation, corrosion or imbalance.
If liquids are to be atomized to a finest possible droplet spray, not only high-pressure single-fluid nozzles only loaded with the liquid to be atomized are used, but also and frequently so-called compressed-gas-assisted dual-fluid nozzles. In these nozzles, the liquid is sprayed with the aid of a compressed, gas, e.g. compressed air or compressed steam as the first gaseous fluid, into a second gaseous fluid, e.g. into flue gas.
Definitions
In the interests of linguistic simplification, the following will use in many cases the designation “compressed air” to designate the first gaseous fluid, with the designation “compressed air” including the use of compressed gas or compressed steam with substantially any required chemical composition. Furthermore, the second gaseous fluid is as a rule referred to as flue gas, the use of the designation “flue gas” including any other gaseous fluid that is possibly solids-laden in addition.
The description of the invention concentrates on the complicated case of the compressed-air-assisted dual-fluid nozzle. The invention is however also applicable to single-fluid pressurized atomizer nozzles, provided the latter are designed as multi-hole or cluster nozzles.
Operational Problems in Nozzles, and Weaknesses of Laboratory Testing:
Together with the energy consumption required for atomization, the characteristic of the created droplet spray is of crucial importance. In this connection, the following problems must be mentioned: the measurement of the droplet distribution in the spray created with a nozzle generally takes place under ideal boundary conditions in fluid mechanics laboratories. The boundary conditions prevailing in large technical facilities are in some cases considerably falsified as a result; for example, the dust content of the flue gas and the loading of the flue gas with easily condensable gases is not simulated in the laboratory. For that reason, the results obtained in the laboratory can only be transposed to a limited extent onto long-term operation in large systems. The easily condensable gaseous constituents of flue gas are in particular sulphur trioxide or sulphuric acid. But in the absence of sulphuric acid, falling below the steam dewpoint can already lead to considerable problems with deposit formation. While the sulphuric acid dewpoint temperature can for example be between 100° C. and 160° C., the steam dewpoint temperatures in flue gases can frequently be between about 45° C. and 65° C. Since with dual-fluid nozzles a comparatively cold fluid is sprayed into the flue gas as a rule, the surface temperature of the nozzle lance and the nozzle head, in particular also that of cluster nozzle heads, is considerably lower than the dewpoint temperatures of the stated flue gas constituents. Liquid condensing from the flue gas at the nozzle lance and nozzle head can chemically react with the particulate constituents of the flue gas, the airborne dusts. It is thus easy to see that airborne dusts with a high quicklime (CaO) content react with the flue gas's sulphur trioxide content condensing as sulphuric acid (H2SO4) to form gypsum (CaSO4), so that hard and firmly adhering deposits can build up. But if the steam dewpoint is not reached at the lance or nozzle surface, not even a sulphuric acid content of the flue gas is required. Even a low sulphur dioxide content is sufficient for the buildup of hard deposits if the airborne dusts contain CaO or MgO, for example. A deposit formation is also possible if only steam is condensed and the condensate sets with deposited airborne dusts.
If however deposits grow in the area of the nozzle outlet openings, it can hardly be avoided that droplets from the spray are deposited onto these deposits and that liquid films form here, as is described in more detail in the discussion of FIG. 1. Comparatively large secondary droplets separate from these liquid films in the range of low shear tension forces. Whereas with a modern dual-fluid nozzle maximum droplet sizes of, for example, 20 to 100 μm are obtainable in principle, the droplets separating from the liquids films can easily have diameters of 500 to 3000 μm. For droplets of such a size, the dwell time even in large technical facilities is much too short even for an only approximately complete evaporation to succeed. Inadmissibly high moisture contents of the product reaching the following components of the facility can result. The insidious thing here is that the deposits on the nozzle head generally only after some time develop sufficiently to exert any severely disruptive influence on the droplet size distribution. Whereas very good results are obtained in a system fitted with new nozzles, over time the operation can be considerably impaired once the deposits have grown thicker.
There is therefore considerable interest in largely preventing deposits on nozzle lances in the close vicinity of nozzles and on the nozzles themselves.
In the case of nozzles with a single outlet hole, deposits can be prevented in a known way using a sheath air device, see for example the international patent publication WO 2007/098865 (PCT/EP 2007/001384). In this case, air is passed with a comparatively low primary pressure, e.g. about 40 mbar, to the nozzle head through a sheathing tube enclosing the actual nozzle lance, and placed around the droplet jet exiting from the nozzle at a comparatively low speed as a sheath air jacket shielding against the flue gas. A deposit formation at the individual nozzle hole can thus be largely ruled out. Even on the nozzle lances, deposit formation is largely suppressed. The latter can be attributed to the fact that the sheath air layer in the outer pipe represents a thermal insulation from the cold nozzle lance, so that the outer skin of the sheathing tube takes on approximately the flue gas temperature, thus preventing any dew formation by flue gas constituents in most cases.
In conventional nozzles with several outlet holes or in the case of cluster nozzles, the supply of the nozzle head area with sheath air causes major problems, as is explained in the following. In such nozzles according to the prior art, the distance between the individual passage openings is very large, as can be seen for example in FIGS. 1 and 2. Every single nozzle acts as a jet pump: it sucks in gaseous fluid, e.g. flue gas, from the environment and mixes it into the spray jet. This gaseous fluid thus flows partly over the cold front surface of the nozzle towards the passage opening, and accordingly the growth of deposits is possible here, at any rate when the gaseous fluid is flue gas. But even if no flue gas reaches the cold front surface of the nozzle, deposit formation can result over time. In this case, the deposits are created from the constituents of the liquid to be atomized itself. This is as a rule not a solids-free liquid, for example fully demineralized and microfiltered water, but process make-up water contaminated with dissolved substances. As shown in FIG. 1, recirculation vortices 17 can be generated by the nozzle jet and return small droplets to the front surface of the nozzle. If the liquid has an opportunity to evaporate here, even if only partly, the constituents automatically grow as deposits.
For a nozzle with several outlet holes, this is shown for example in FIG. 1, which also shows the liquid film 12 on the deposit and also the large secondary droplets 13 created. The critical factor in such nozzles with several outlet holes is in particular the central area, which frequently has no outlet hole for design reasons. A first step to improve the boundary conditions would thus be to revise the design of a multi-hole nozzle to the effect that a central outlet hole is possible. By arranging a sheath air nozzle according to the prior art, the deposit formation from flue gas constituents can be prevented in such nozzles with several outlet holes. However, a relatively large sheath air volume flow is required if a deposit formation on the front surface of the nozzle is to be dependably thwarted. It is of course not desirable to supply an unnecessarily large amount of sheath air to the nozzle jet, since it is of course not the sheath air but the flue gas which is to be cooled by droplet evaporation. There is thus a strong interest in keeping the nozzle front surface susceptible to deposit formation as small as possible and to reduce as far as possible the distance between the individual nozzle outlet holes. In the case of nozzles according to the prior art this is not possible, since for this purpose the outlet holes must be arranged closely around the central axis, as shown in FIG. 1. Then however the inflow to these nozzle holes is very unfavourable and involves high pressure losses and flow separation in the outlet holes, and an unsatisfactory atomization.
The situation with cluster nozzles according to the prior art is even more critical, as shown in FIG. 2. Here it would be necessary to work with a very large amount of sheath air and with a sheath air nozzle head of complex design if a deposit formation from flue gas constituents is to be reliably prevented. A deposit formation from the solids content of the liquids to be atomized cannot yet however be prevented with this.